Hornady Magnum Rimfire, commonly known as the .17 HMR, is a rimfire rifle cartridge descended from the .22 Magnum. It was developed by necking down the .22 Magnum case to take a .17 caliber (4.5 mm) bullet. Commonly loaded with a 17 grain (1.1 g) bullet, it can deliver muzzle velocities in excess of 2550 ft/s (775 m/s).